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Thread: Radio Controlled Adjustable Limit Straps

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Old 04-10-2014, 08:28 PM   #1
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Default Radio Controlled Adjustable Limit Straps

IF YOU DON'T LIKE READING, SKIP TO THE VIDEO AT THE END OF THE POST.

One of the reasons that rigs (particularly those with droop suspensions) tend to topple over backwards when attempting to climb steep faces is suspension buckle. Initially, as the rig begins to climb, the front wheels pull and the wheelbase (distance between front and rear axles) elongates as the suspension flattens. Eventually, the rear wheels reach the face and when they bite at the transition from horizontal to vertical, they tend to pivot the rig over backwards, thereby lifting the front wheels away from the face. When this occurs, the front wheels are no longer able to pull and the suspension collapses or buckles and the wheelbase is shortened. In the process, the CG of the rig is shifted out away from the face. This is why, even if you are able to reverse and keep the rig from flipping over backwards, the rig seems to feel even more unstable and likely to flip over backwards when you resume climbing. The crappy image below illustrates suspension buckle.



In the upper drawing, the front wheels are pulling and the wheelbase is elongated, which pushes the CG toward the face and keeps it above the rear axle. If the front wheels lose traction, the suspension collapses, the wheelbase shortens, and the CG is shifted out away from the face as depicted in the lower drawing. When this happens, the rig is much more likely to flip over backwards.

One way to reduce/prevent suspension buckle, is to use a limiting strap. Full sized rock crawlers use limiting straps to keep the suspension from over-extending, which can cause shocks to come apart among other things. All a limiting strap is is a piece of webbing that runs from the chassis to the axle. In our case. we would install limiting straps to the front and rear suspension to keep the suspension flattened and snug. I tried this out by running a piece of para-cord from the rear axle, over the top of the chassis to the front axle and back, drawing it taught with a rolling hitch. My rig then crawled straight up a 65 degree slab that had been tossing us all afternoon.

But this introduces a new problem. That is, it prevents the suspension from flexing in the way that it needs to in order to enable the rig to pull itself up over the edge. Basically, the limiting straps would cause the skid plate to drag and get stuck on the lip of the ledge. This is illustrated below.



In the upper drawing, the limiting straps keep the suspension from flexing to accommodate the ledge. In the lower drawing, the suspension flexes to keep both the front and rear wheels in contact with the terrain.

Despite this shortcoming of the limiting straps, I think they can still be used to great advantage. However, as I thought about this problem with using limiting straps, it gave me an idea. That is, what if we had the ability to apply and release tension to/from the limiting straps as needed. Imagine if we had a winch on the front axle and we ran the hook up over the top of the chassis and fastened it to the rear axle. As the cable is reeled back in, it would flatten the suspension, elongate the wheelbase, and lower the CG. Conversely, as the winch is unspooled and tension in the cable is released, the suspension would be released. Tonight, I put together a crude assembly to test this idea. I think the results speak for themselves as you can see in the video GMade R1 Adjustable Limit Strap - YouTube
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Old 04-10-2014, 09:15 PM   #2
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Default Re: Radio Controlled Adjustable Limit Straps

There was a guy running something like this at Nationals in 2007.
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Old 04-11-2014, 06:27 PM   #3
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Default

At one point a lot of the buggies ran suck down winches on either end for the same effect in the 1:1 world.

Seems it should work for you, but maybe a winch in each end to have independent control might be better?

Still it's an interesting concept. Nice job
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Old 04-11-2014, 10:08 PM   #4
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Default Re: Radio Controlled Adjustable Limit Straps

Quote:
Originally Posted by tgh117 View Post
At one point a lot of the buggies ran suck down winches on either end for the same effect in the 1:1 world.

Seems it should work for you, but maybe a winch in each end to have independent control might be better?

Still it's an interesting concept. Nice job
Thanks! It would be cool to have independent control of front and rear. Really the only drawback to this setup is the added weight. I'm not sure if the potential gains will be worth the additional weight and complexity, much less the time spent to optimize this setup. It seemed to work really well on the rock pile this afternoon, though.
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Old 05-03-2014, 03:16 PM   #5
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Default Re: Radio Controlled Adjustable Limit Straps

I made an adjustable limit strap for my rear suspension. When I tighten it all the way, it doesn't really restrict my articulation much at all and it enables me to climb really steep faces that I could not climb before. The face in this video (GMade R1 Climbs Super Steep Face - YouTube) used to be impossible. Now it is almost effortless.
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